1. Field of the Invention
The present invention relates to a multiplexing/demultiplexing method of at least two encoded digital video data streams for transmission and storage, and more particularly to a multiplexing/demultiplexing method which provides a selective mode for superimposing sub-images on main images.
2. Description of the Prior Art
Digitized video signals are compressed and encoded for transmission and storage at a reduced bit rate. The motion-compensated transform coding algorithm is currently the most widely used method and has been adopted as the key technique by major international standards such as H. 261, MPEG1 and MPEG2 for compressing digital video sequences. A wide variety of literature on this coding method is available, an example of which is published in "The MPEG Video Compression Algorithm," Signal Processing, Image Communication, Vol. 4 No. 2, pp. 129-140, April 1992. Briefly, the algorithm works in the following manner. A current picture is divided into adjacent blocks each containing a matrix of pixel data. For each block, a motion vector is detected by comparing the current block to a reference picture to find the best-matched prediction among possible blocks in the reference picture. The difference between the predicted block and the current block, i.e., the prediction error, is further compressed using an orthogonal transformation to remove spatial correlation. The transformed coefficients are quantized by a quantization parameter to discard the less important information. Finally, the motion vectors and quantization parameters are combined with the quantized coefficients, the location information of each block as well as the other side information for transmission or storage by using fixed or variable length codes.
In the image coding scheme described above and in other schemes, the encoded digital video data, hereafter referred to as data stream, are in Binary ("0" or "1") presentation. The data stream is usually packetized, i.e., arranged in units of packets, and then multiplexed with the audio data stream and/or other data streams for transmission or storage. An identification tag indicative of each of the data stream is appended to every packet for distinguishing the various data streams. U.S. Pat. No. 5,202,886 describes a system for multiplexing/demultiplexing of digital television information, in units of packets, for transmission and reception.
In some applications, such as broadcasting and teleconferencing, there are needs where the main image is supplemented or assisted by a sub-image or a group of sub-images. Examples of the sub-images are closed captions, sign language interpretation for the handicaps, drawings and writings and the image of other speakers during teleconferencing between two points.
In conventional television broadcasting, sub-images for sign language interpretation are superimposed on to the main image before transmission. A similar procedure which can be adopted for digital television system. This method, however, prevents the viewers from selectively displaying the sub-images. In other words, a viewer can not choose whether or not to display a sub-image.
Television receivers that offer a "picture-in-picture" function, where the image received from a second channel is superimposed on to the image of a first channel, requires separate tuners and demodulators for the first and second channels (see for example U.S. Pat. No. 4,665,438). In a similar manner, a digital television receiver can be equipped with multiple decoders for decoding more than one program. In teleconferencing systems, multiple decoders are set up for decoding more than one image.